This invention pertains to the field of methods for cleaning and surface treating aluminum products to improve their brightness. More particularly, the invention pertains to an improved, more efficient method for surface treating aluminum wheel products made by forging, casting and/or joining practices. Such wheels are suitable for automobiles, light trucks, heavy duty trucks and buses. This invention may also be used to surface treat aerospace wheels and other aerospace components.
Present surface treatments for bright aluminum products involve a plurality of separate steps including: cleaning, deoxidizing, chemical conversion and painting. Some of the foregoing process steps typically incorporate surface active agents and/or corrosion inhibitors. The final painting step for many aluminum products is a polymeric clear coat applied in either a liquid or powder form. All these processes rely on the availability of bright aluminum surfaces for starting. Part of the overall success of these surface treatments hinges on minimizing initial brightness degradation during application of the known chemical treatments described in more detail hereafter.
Disadvantages with such prior art processes include:
1. They required a starting bright aluminum surface. The processes did not induce any brightness themselves.
2. The chemical treatment (i.e. cleaning, deoxidizing and chemical conversion) and painting steps typically reduced the brightness of these aluminum surfaces. That, in turn, detrimentally impacted the initial properties of aluminum products made thereby.
3. Many chemical treatment and painting processes were applied to enhance: (a) the adhesion of subsequent coatings to these aluminum products; and (b) the corrosion resistance performance thereof. For any given product, a compromise had to be reached between greater brightness and greater durability.
4. From a manufacturing standpoint, past processes involved a large number of steps requiring relatively high levels of employee involvement to assure consistency and quality. That translates into high operating and production costs.
5. While maximum corrosion resistance may be achieved with hexavalent chromium, that component should be avoided because of its detrimental environmental and health risks.
Numerous processes for cleaning, etching, coating and/or surface treating aluminum products are known. They include: U.S. Pat. Nos. 4,440,606, 4,601,796, 4,793,903, 5,290,424, 5,486,283, 5,538,600, 5,554,231, 5,587,209, 5,643,434 and 5,693,710.
In U.S. Pat. No. 5,290,424, the disclosure of which is fully incorporated by reference herein, reflectivity of a particular product, decorative lighting sheet made from 5000 or 6000 Series aluminum alloys, was improved. In the process of that patent, a flexible coating was applied to that lighting sheet product. The present invention, by contrast, is not limited to just sheet product. It can also be used to surface treat aluminum forgings and castings, especially those made from Al-Mg alloys, Al-Mg-Si alloys, Al-Si-Mg alloys and/or copper-containing variants of the latter two alloys. Furthermore, the desired end result of THIS inventive method is a coating that is brittle and hard to scratch (abrasion resistant), for greater optical clarity (including brightness and corrosion resistance), unlike the focus of the more flexible coating patent described above.
The present invention imparts brightness to the surface of aluminum products, especially vehicle wheels, while improving the adhesion, soil resistance and corrosion resistance performance of such products. This invention accomplishes the foregoing property attributes through a manufacturing sequence that involves 25% fewer steps thereby reducing overall production costs. The invention combines two of the more costly known surface treatment steps, those of surface brightening and cleaning, into one step. At the same time, the method of this invention employs more user friendly components that pose no immediate or long term risks to operators or the environment. Finally, because of the chemical nature of this process, resulting end products exhibit a higher abrasion resistance.
The new method of this invention consists of:
Main Step 1. A single chemical treatment, the composition and operating parameters of which are adjusted depending on whether the preferred products to be treated are made from an Al-Mg, Al-Mg-Si or an Al-Si-Mg alloy. This chemical treatment step imparts brightness to the aluminum being treated while yielding a chemically clean outer surface ready for subsequent processing. This step replaces previous multi-step buffing and chemical cleaning operations. On a preferred basis, this chemical brightening step uses an electrolyte with a nitric acid content between about 0.05 to 2.7% by weight. It has been observed that beyond 2.7 wt % nitric acid, a desired level of brightness for Al-Mg-Si-Cu alloys cannot be achieved. On a preferred basis, the electrolyte for this step is phosphoric acid-based, alone or in combination with some sulfuric acid added thereto, and a balance of water.
Main Step 2. The second main step is to deoxidize the surface layer of said aluminum product by exposure to a bath containing nitric acid, preferably in a 1:1 dilution from concentrated. This necessary step xe2x80x9cprep""sxe2x80x9d the surface for the oxide modification and siloxane coating steps that follow.
Main Step 3. The third main step of this invention is a surface oxide modification designed to induce porosity in the surface""s outer oxide film layer. The chemical and physical properties resulting from this modification will have no detrimental effect on end product (or substrate) brightness. Like main step 1, the particulars of this oxide modification step can be chemically adjusted for Al-Mg-Si versus Al-Si-Mg alloys using an oxidizing environment induced by gas or liquid in conjunction with an electromotive potential. Surface chemistry and topography of this oxide film are critical to maintaining image clarity and adhesion of a subsequently applied polymeric coating. One preferred surface chemistry for this step consists of a mixture of aluminum oxide and aluminum phosphate with crosslinked pore depths ranging from about 0. 1 to 0.1 micrometers, more preferably less than about 0.05 micrometers.
Main Step 4. Fourthly, an abrasion resistant , siloxane-based layer is applied to the aluminum product, said layer reacting with the underlying porous oxide film, from above step 3, to form a chemically and physically stable bond therewith. Preferably, this siloxane coating is sprayed onto the substrate using conventional techniques in which air content of the sprayed mixture is minimized (or kept close to zero). To optimize transfer onto the aluminum part viscosity and volatility of this applied liquid coating may be adjusted with minor amounts of butanol being added thereto.
The foregoing method steps of this invention eliminate filiform corrosion while maintaining an initial brightness of the aluminum product to which they are applied. In some instances, the invention also imparts brightness to the product while yielding a chemically clean surface in fewer steps thereby reducing overall production costs. Finally, this invention imparts some degree of abrasion resistance, a major requirement for various aluminum products such as vehicle wheels made by forging, casting or other known or subsequently developed manufacturing practices. It accomplishes all of the foregoing without the use of environmentally risky or health threatening components.